1. Field of the Invention
The present invention relates to apparatus and methods for measuring the blood n through various tissue segments of the human body. In particular, the invention relates to apparatus and methods to measure the electrical impedance of a segment of a human body caused by changes in the volume of blood in the segment.
2. Description of the Related Art
The electrical bioimpedance of a segment of a human body depends upon a number of factors, one of which is the quantity of blood and the conductivity of the blood. Measuring the electrical bioimpedance of the segment is a convenient means for non-invasively determining the blood perfusion of the various tissues in the segment. By measuring the magnitude of the unchanging components of the bioimpedance as well as the rate and amplitude of changes in the bioimpedance caused by blood flow generated by the pumping action of the heart, several important cardiac parameters can be calculated and used to determine the condition of the heart.
When measuring the electrical bioimpedance of a body segment, such as the thorax, the primary interest is in changes in the electrical bioimpedance caused by the periodic increases and decreases in the quantity of blood in the segment caused by the periodic pumping action of the heart. The thoracic area of the human body is typically the principal area where measurements of the cardiovascular bioimpedance occur because of the presence of large blood vessels that have significant changes in blood quantity throughout the cardiac cycle. However, changes in the thorax during respiration also cause changes in the electrical bioimpedance of the thorax and thus cause major difficulties in measuring the electrical bioimpedance of the cardiovascular activity in the thoracic area. The changes in the bioimpedance due to respiration are approximately an order of magnitude greater than the changes in electrical bioimpedance caused by the heart and are superimposed over the smaller cardiovascular bioimpedance changes to form a composite bioimpedance signal.
Some devices presently being used require voluntary apnea to take a measurement of the cardiovascular component of bioimpedance. However, this requirement of apnea makes it extremely difficult, if not impossible, to measure the cardiovascular bioimpedance in many instances. Often voluntary apnea cannot be performed because the person whose bioimpedance is being measured is unconscious, under anesthesia, or ill. Further, even when voluntary apnea may be performed the undisturbed cardiovascular electrical bioimpedance can be measured only for a short time.
Moreover, it is difficult, if not impossible, to completely separate the cardiovascular bioimpedance signals from the respiratory bioimpedance signals by using filtering. A common approach has been to use the first derivative of the composite of the bioimpedance signal. This reduces the magnitude of the problem because the derivative reduces the lower frequency/higher magnitude respiratory part of the bioimpedance signal. One such method, disclosed in U.S. Pat. No. 4,450,527, calculates and uses a sliding average of the maximum rate of impedance change over four heart beats to further offset the effects of respiratory bioimpedance. Even with the improved methods presently available, respiratory changes in the bioimpedance continue to interfere with the accurate measurement of cardiovascular bioimpedance.